Oil well casing wash composition



Aug. 21, 1956 ROHRBACK ET AL 2,759,891

OIL WELL CASING WASH COMPOSITION Filed April 29, 1953 l l i 6 A AINVENTORS G/LSON H. ROHRBACK JOSEPH F CH/TTUM United States atent OILWELL CASING WASH COMPOSITION Gilson H. Rohrback, Seattle, Wash., andJoseph F. Chittum, Whittier, Caliii, assignors to California ResearchCorporation, San Francisco, Calif., a corporation of DelawareApplication April 29, 1953, Serial No. 351,833

2 Claims. (Cl. 252-855) This invention relates to a composition andmethod for inhibiting the corrosion of oil well casings.

Although relatively little attention has been given to corrosion failureof oil well casings in the literature, this problem is ratherwide-spread. A number of theories have been proposed to account forcasing corrosion and some attempts to control it have been made on thebasis of the theories. Among the theories heretofore advanced to explaincasing corrosion have been suggestions that the corrosion was due tobacterial action, due to electric currents flowing through the casing,due to localized corrosion at metal flaws, due to particularcompositions of formation water, due to corrosion by selfpotentialcurrents and due to interzonal migration of salt water. So far as isknown, no successful method of controlling casing corrosion has beendeveloped on the basis of any of these theories. Meantime, casingfailure due to corrosion continues to be a serious problem in many oilfields. Repair of casing failure is difficult at best and if the failureescapes detection for an appreciable period of time, the well ceases toproduce, and production may not be resumed, even though the casing berepaired, necessitating abandonment of the well.

A number of instances of casing failure due to corrsion in Californiaoil fields have been carefully studied. From these studies it wasconcluded that corrosive attack upon ferrous metal casings is rapid andlocalized when different points of the exterior of the casing are incontact wit-h fluids differing markedly in pH or oxygen content.

Current practice in completing oil wells is such that the fluidsresident in the annular space between the outer wall of the casing andthe formation will commonly exhibit marked differences of pH atdifferent points along the casing wall. When a well has been drilled tothe desired depth and is ready for completion, the casing is full ofdrilling mud at the commencement of the cementing operation. It israther common practice to introduce an aqueous solution of an alkalimetal polyphosphate (commonly referred to as a pyro-wash liquid) intothe casing before the cement is introduced. After the introduction ofthe aqueous polyphosphate solution, cement is introduced and forced downthe casing. As the cement travels down the casing, there are three moreor less distinct fluid phases in the casing, the bottom phase beingdrilling mud, the middle phase being an aqueous solution of apolyphosphate, and the upper phase being the cement slurry. When thenecessary amount of cement has been introduced, drilling mud isintroduced behind the cement phase and forced down the casing. Thebottom mud layer, the pyro-wash layer, and the cement layer flowdownwardly through the casing and around the lower circumference of thecasing and upwardly through the annulus between the outer casing walland the formation. When the cement has been forced into suitableposition between the outer wall of the casing and the formation, it isallowed to set. The annular space between the outer wall of the casingand the formation when the well is completed will commonly contain alayer of drilling mud in the upper part of the annulus, below thedrilling mud a layer of aqueous polyphosphate solution, below thepolyphosphate solution a cemented zone, and below the cemented zone alayer of drilling mud extending to the lower extremity of the casing.

Drilling muds normally have pH values ranging from about 8.5 to about12. The aqueous polyphosphate solution normally has a pH of about 12 incontact with cement. A difference in pH between the drilling mud layerand the aqueous polyphosphate layer present in the annular space betweenthe outer casing wall and the formation is commonly so large as to causerapid electrochemical corrosion of the casing which continues to thepoint of easing failure. In cases in which a high pH mud is used incombination with the polyphosphate wash, there will be little differencein pH between the mud and the wash solution, but formation waterordinarily having a pH of 7.5 very commonly breaks through the mud layerat one point or another andmakes contact with the casing so that largepH differences exist between the pyrowash solution and the formationwater and between the mud, if it is a high pH mud, and the formationWater.

It has now been found that casing corrosioncan be very markedly reducedby incorporating a small amount of stannous chloride in the pyro-washsolution. Pyrowash solutions are normally wash solutions ofpolyphosphate such as tetrasodium pyrophosphate, sodiumtripolyphosphate, sodium hexa-meta-phosphate, and the like, in which thepolyphosphate content. is from, about 02% to 2% by weight. Large amountsof polyphosphate up to the point of saturation of the water may beemployed. To inhibit casing corrosion from 0.05 to 5% by weight ofstannous chloride is dissolved in the pyrowash solution.

While stannous chloride is preferred in compounding the wash solution ofthe. invention, it is the stannous ion which is the effective inhibitor.Accordingly, other stannous compounds which are soluble or dispersiblein water, such as stannous sulfate, stannous nitrate, stannoushydroxide, etc. can be substituted for the stannous chloride and arefound essentially equally effective.

Laboratory studies of the nature of casing corrosion and of compositionsand methods of controlling it were made in the apparatusdiagrammatically illustrated in the appended drawing. In the drawing,container 6 is a glass jar having about 4 gallons capacity. Electrode 1was made up of 3 short sections of concentric pipe welded together. Theweld areas were covered with a plastic paint to eliminate undesirablecouples from these areas. The total area of this electrode was about 330square inches. Electrode 4 is a small strip of iron having a surfacearea of approximately 2 square inches. Electrode 1 is welded to iron rod2 which is attached to adjustable support 5 so that the position ofelectrode 1 in the jar can be adjusted at will. Electrode 4 is connectedto rod 2 by wire 3. In making the experimental tests two fluids wereintroduced into jar 6. The fluids differed from each other in both pHand density. In each test, jar 6 was filled with the denser of the twofluids employed to level AA and then the less dense of the two fluidswas introduced into jar 6, filling the jar to about level BB. Electrode1 was positioned in the jar so that it was entirely surrounded by thefluid of higher pH. Electrode 4 was positioned in the jar so that it wasentirely surrounded by the fluid of lower pH. The electrodes were thenleft in these relative positions for a period ordinarily of 14 days, atthe end of which electrode 4 was removed and weighed to determine itsloss of weight. In order to determine the weight loss of electrode 4 dueto electrochemical action alone, a second small iron strip approxi:

mately identical with electrode 4 in size and shape was 3 suspended fromthe wall of the jar in the same fluid which surrounded electrode 4. Atthe end of the test period the weight loss of electrode 4 and the weightloss of the second metal strip were determined and the differencebetween these two losses was the loss of weight of elec- 5 trode 4 dueto electrochemical action. Results of a series of experiments are setforth in the table below. The muds employed in the tests were typicalcommercial drilling muds. The formation water was a typical aqueousefliuent from a California well and the pyroflnid 10 was an aqueoussolution of tetrasodium pyrophosphate such as is commonly employed forwashing the casing prior to introduction of the cement and which iscommonly left in the annulus between the outer casing wall and theformation when the well is completed. The etfect of 15 various additiveson the weight loss due to electrochemical corrosion is shown in thetable.

TABLE containing wash solution diffuses into the mud. Other naturaltannin compounds such as chestnut extract, and the like, may besubstituted for the quebracho.

A suitable pyro-wash solution can be prepared by dissolving 18 pounds oftetrasodium pyrophosphate and 5.4 pounds of stannous chloride in 10barrels of water. This Wash solution can be improved in respect to itseffect on mud viscosity by the addition of about 18 pounds of quebrachoto the 10 barrels of solution.

We claim:

1. An oil well mud cake wash solution comprising an aqueous solution ofan alkali metal polyphosphate containing a small amount from about 0.05to about 0.15% by weight of stannous chloride.

2. An oil well casing wash solution suitable for use in filling at leasta part of the annular space between the casing and the formation in acompleted oil well comof treated and untreated fluids Electrode 1Electrode 4 Measured Weight Loss from Elec- Test N 0. Fluid Additive pHFluid Additive pH trade 4 Due to Current (Milligrams) Pyro N one 12. 3Mud None 8. 5 968 Pyro Nonem. 12.3 Mud Sodium Ohromate- 8. 4 522 MudNone 8. 5 Pyro SnC 12.1 0 Formation Water. None... 7. 1 Pyro 12. 2 n MudSnOhl2. 2 Pyro l2. 2 0

2Mud contained sodium chromate0.02 percent. 3-Pyro contained SnCl:-0.15percent, and quebrach00.5 percent. 4-Pyro contained SI1C120.15 percent,and quebracho0.5 percent.

5-Both mud and pyro contained SnClz-OJS percent, and quebraeho0.05percent. When pyro was treated with SnOh, the cell was set up withelectrode 4 in pyro solution.

prising an aqueous solution of an alkali metal polyphosphate containinga small amount from about 0.05 to about 0.15% by weight of stannouschloride and from 40 about 0.1 to 3% by weight of quebracho.

References Cited in the file of this patent UNITED STATES PATENTS2,322,484 Stuart June 22, 1943 2,490,291 Wrightsman Dec. 5, 19492,580,765 Hall et a1. Jan. 1, 1952

1. AN OIL WELL MUD CAKE WASH SOLUTION COMPRISING AN AQUEOUS SOLUTION OFAN ALKALI METAL POLYPHOSPHATE CONTAINING A SMALL AMOUNT FROM ABOUT 0.05TO ABOUT 0.15% BY WEIGHT OF STANNONS CHLORIDE.